How many viable humans are possible?

I'm not sure how the author derived this number because it's not taking into account the different alleles that exist of each gene. For this exact reason, we also don't know which alleles are compatible with each other.

Would you say 70 Trill might be a high number? I think Neil DeGrasse Tyson said once that maybe 1/10 or less of a percent of all possible human DNA combinations have been played out in human history so far. Basically we ain't near done.

Assuming no crossing over occurs in during meiosis, there are 223 = 8,388,608 different chromosome combinations.



On top of all the possible chromosome combinations, crossing over also adds another source of variation. Crossing over can produce a single chromosome that contains a new combination of genetic information from different parents, a result called genetic recombination.



Coupling this with someone else who can a completely different line of alleles for each gene, it'd be practically impossible to get another human that's exactly the same as you.

So is there a way to calculate how many DNA combinations aren't viable if we know all that are?

I am still trying to understand how well geneticists understand DNA sequences that does harm versus good.

Let's start over again to make sure we're both on the same wavelength in terms of what we know.

All humans beings have the SAME genes. Assume human DNA codes for TEN genes (for simplicity). That means both you and I have ten genes; if you had 11 genes, then technically you wouldn't be "human".

Now, the reason you and I look different is because for each one of your genes, their "alleles" are different that mine. For example, assume gene 1 codes for skin color. My allele would lead to an olive skin phenotype, while yours leads to, say, white (assuming).

The problem with answering your question about which combination of genes are lethal is impossible to my knowledge is because we don't know which allele combinations are lethal. For instance, if I produce a gamete that encodes olive skin color, and you produce a gamete that encodes white skin color, we don't have any theoretical data that tell us if combining these alleles through sex reproduction leads to a problematic zygote. Without that data, there's really no pathway forward.

I don't mean necessarily fatal, but harmful such as mental disabilities or two genes that have effects that work against each other.

You mean two alleles of different genes that work against each other? Sometimes you don't need two or more genes to create a fatal scenario. For example, cystic fibrosis, Huntington's disease, sickle-cell anemia, and brachydactyly are all caused by a faulty allele. In the case of cystic fibrosis, both parents of an offspring must contribute the bad allele for the child to have the condition. In the case of Huntington's disease, the offspring only needs one bad allele, as the condition is dominant, not recessive like in cystic fibrosis. Then you have alleles that are only lethal under specific environmental conditions. One example of a conditional lethal allele is favism, a condition that causes the carrier to develop hemolytic anemia when they eat fava beans. Again, I don't think we have an extensive list of all lethal alleles. It's hard enough knowing when one allele from a single gene leads to harmful outcomes; factoring two or more genes makes it much more complicated.